Cholesterol is a major component of atherosclerotic plaque. Cholesterol accumulation within atherosclerotic plaque occurs when cholesterol influx into an arterial wall exceeds cholesterol efflux. Increased influx of cholesterol into the arterial wall is accompanied by an increased influx of monocytes/macrophages, which absorb oxidized aggregated low density lipoproteins (LDL) and store the cholesterol esters.
Current strategies to reduce coronary heart disease are primarily directed at reducing the influx of cholesterol into the arterial wall by lowering LDL cholesterol concentration. While lowering of plasma LDL levels offers some protection from coronary heart disease, the protection is not complete. As a result, there is an interest in strategies aimed at enhancing cholesterol efflux from the arterial wall and promoting its transport to the liver for excretion.
Cholesterol circulating in the blood is carried by plasma lipoproteins. Plasma lipoproteins are classified into groups according to size. Of these, the low density lipoprotein (LDL) and high density lipoprotein (HDL) are primarily the major cholesterol carrier proteins. The protein component of LDL, apolipoprotein B (Apo B), constitutes the atherogenic component. Apo B is not present in HDL. HDL includes apolipoprotein A-1 (Apo A-I) and apolipoprotein A-II (Apo A-II) as well as other apolipoproteins.
Various forms of HDL have been described on the basis of electrophoretic mobility and include two major fractions: a first fraction with α-HDL mobility and another fraction referred to as pre-β HDL. Pre-β HDL is thought to be the most efficient HDL subclass for inducing cellular cholesterol efflux. Pre-β HDL fractions includes Apo A-I, phospholipids and free cholesterol. Pre-β HDL are considered to be acceptors of cellular free cholesterol and are believed to transfer free and esterified cholesterol to α-HDL.
Two pathways have been proposed to describe the movement of cholesterol from a plasma membrane to acceptor particles such as pre-β HDL. In the “aqueous diffusion pathway,” cholesterol molecules spontaneously desorb from cell membranes and are then incorporated into acceptor particles (pre-β HDL) after traversing the intervening aqueous space by diffusion. It is believed that the aqueous diffusion pathway does not require interaction with specific cell receptors.
The second model, referred to as the “microsolubilization pathway,” involves the interaction of HDL (presumably an Apo A-I interaction) with a cell surface binding site. The HDL induces an intracellular signal leading to translocation of cholesterol from intracellular sites to the plasma membrane. The physiological acceptors or carriers for the translocated cholesterol are nascent HDL particles, including α-HDL and pre-β HDL.
Cholesterol that is transferred to nascent HDL particles is esterified by lecithin-cholesterol acyl transferase (LCAT) to cholesteryl esters. These esters are hydrophobic and tend to move into the core of the HDL particle. The HDL cholesteryl esters may return or be delivered to the liver and are excreted from the liver into bile, either directly or after conversion to bile cells.
It is believed that α-HDL and pre-β HDL particles, the primary acceptors or carriers for translocated cholesterol, do not occur in the same relative fractions as nascent HDL particle in the blood stream of an adult human. Thus, the carrier potential of each fraction is believed to be inversely proportional to its relative fraction of the total HDL quantity. In other words, the fraction with the highest carrier potential (pre-β HDL) occurs in the smallest overall amount in vivo.